Comparing the seoi-nage skill of elite and non-elite judo athletes

Seoi-nage performance requires a high level of skill and proficiency. The aim of this study was to compare the motor planning, regulation, and control skills of elite versus non-elite seoi-nage judo athletes. Twenty subjects (10 elites and 10 non-elite) performed the three-phase seoi-nage skills of unbalancing, positioning, and throwing while an optical motion capture 3D camera monitored their shoulder, pelvis, hip, and knee joint movements to calculate their force magnitude and direction. Elite athletes performed better than non-elite athletes in terms of the shoulder (247.4° vs. 208.3° in Event 4) and pelvic (235.4° vs. 194.4° in Event 4) rotation, tilt angle (15.13° vs. − 0.74° in Event 4) characteristics, as well as hip (136.1° vs. 125.0° in Event 4) and knee joint (124.0° vs. 120.8° in Event 3) flexion–extension angle. Compared to non-elite athletes, elite athletes also showed more controlled force and movement in all bodily areas. These results can help to guide the development of seoi-nage skills as judo athletes advance from the non-elite to the elite level.

Furthermore, judo-specific device is required to/is considered to measure the single-axis pulling force and the amplitude of the force in each phase of techniques in the execution of seoi-nage, as well as measuring kinematics.
Several studies to understand judo skills in soei-nage have been conducted, but little is known to explain the mechanism how athletes with different levels of skills and execution demands perform in the process of motor skill acquistion.It has been reported that the seoi-nage is crucially played in the adaptability of body/balance control when postural stability is perturbed.However, specific kinematic characteristics of seoi-nage particularly associated with skill level are still unknown.
The aim of the present study was to examine the kinematic characteristics of seoi-nage on body/balance control in response to skill level.In this study, the kinematic variables of seoi-nage were simultaneously examined in three dimensions while the force magnitude was measured by suspending a rubber tube for judo training in judo-specific equipment.Our study differs from earlier ones in particular by simultaneously analyzing kinematic features and measuring the force value of seoi-nage using specific equipment.Our hypothesis was that, in a period of seoi-nege, non-elite athletes would demonstrate bigger kinematic deficits than elite athletes do.Consequently, these results can probably be used as factual information to improve the efficiency of judo seoi-nage learning and to further serve as the foundation for precise training recommendations.

Participants
Partcipants in this study were twenty Judo athletes.All athletes were ethnic Korean.On average, elite athletes were 20.9 years old (SD = 0.88; range = 20-22) and had 7.7 years (SD = 2.54) of athletic experiences.On average, non-elite athletes were 27.5 years old (SD = 2.68; range = 20-29) and had 4.7 years (SD = 2.50) of athletic experiences.As shown in Table 1, we assigned them into ether elite athlete group (EG; n = 10) or non-elite athlete group (NG; n = 10).All participants had no history of neurologic disease or musculoskeletal dysfunction and had no prior experience with the experimental task and were not aware of the specific purpose of the study.All participants were right-hand dominant, as determined by self-report prior to the experiment.In this study, the criteria for participants recruitment whether athletes were included are as follows: Elite athletes who had the experience of international competitiions as national representitives for Korea were included in elite athletes group, while non-elite athlete had no experience of international competitions at all.All participants were requested to understand the purpose of this study for voluntary participation.The t-test on the height and weight of athlete participants in both EG and NG showed no significant difference in the height (t = -0.40,p = 0.696) and body weight (t = 1.31, p = 0.206) between EG and NG.In addition, using the G*Power 3.1.9.7 program, the sample was calculated with an effect size of 0.30, a significance level of 0.05, and a power of 0.95 required to test the difference.As a result, a minimum of 19 research participants were required.
The protocol was approved by the Institutional Review Board of Pukyong National University (1041386-2023-HR-39-01) and conformed to the Declaration of Helsinki.All aspects were conducted in accordance with the relevant guidelines and regulations of the institution.experimental Informed consent was obtained from each subject before participation in the experiment.

Apparatus and task
A vector system was used to measure the magnitude and direction of the force.A three-axis load cell sensor was used to measure the magnitude and direction of the force on the Z-axis approximately 9800 N (in 1 ton), X and Y-axis approximately 4900 N (in 500 kg), and nonlinearity (1%).The maximum yield load was 150%.While the Z-axis measured the subject in the horizontal direction, the Y-axis and X-axis measured those in vertical and horizontal directions.We measured the magnitude of force appearing in the X, Y, and Z directions and the sum of the force vectors at a sampling frequency of 200 Hz (Fig. 1).The rubber tube connected to the judo throw analysis device was held with both hands to perform seoi-nage at maximum speed and force.The baseline was similar for the rubber tube and both arms.The equipment was located 80 cm from the ground, and the distance between the participant and the equipment was 150 cm.
We used an optical motion capture camera (Optitrack, Natural Point, Inc. USA) system to analyze the 3-D images recorded during seoi-nage.The subjects' kinematic variables, i.e., shoulder and pelvis rotation angle (counterclockwise rotation + value, clockwise rotation − value), shoulder and pelvis tilt angle (posterior tilt + value, anterior tilt − value), and hip and knee joint angles (extension + value, flexion − value), were measured using 20 cameras.The motion capture camera's frame rate was 120 frames per second, and reflective markers were positioned on the head, shoulder, hip, knee, ankle, and posterior superior iliac spine to measure the performance.A marker was attached to the seventh cervical spine to distinguish the center of the body and on the lower scapula to differentiate the left and right sides.Before the task, a suit with a marker was attached for the 3-D image analysis.We attached the markers to each position and placed them at the baseline to hold the rubber tube connected to the judo throw equipment.Both groups participated in a series of seoi-nages, with two practice attempts.A baseline was set at a distance of 150 cm from the judo throw equipment to identify the performance position.Subsequently, on indicating a ready sign, the participants performed seoi-nage with maximum force and speed.
The task involved performing seoi-nage five times, with a 30-s interval between each attempt.The kinematic data measured using the Optitrack motion capture program were digitized by signal cleaning through C motion's 3-D 5 V professional, and the quantified data were normalized using MATLAB (The MathWorks Inc., R2015b, Version 8.6, Massachusetts, USA).The kinematic characteristic data collected during seoi-nage were classified into four events (Fig. 2).First, the beginning of seoi-nage and first left foot contact time was set as Event 1, following which the left foot rotated and landed, defined as Event 2 (unbalancing phase).Event 3 was set as the point where the shoulder's horizontal rotational angle reached 180° (positioning phase).Event 4 was set as the point when the horizontal rotational angle of the shoulder reached its maximum (throw phase).Event 1 was defined as the motion of stepping forward with the right foot before pulling the judo rubber tube from the stationary position and was excluded from the analysis owing to the absence of pulling motion.The analyzed kinematic variables were the shoulder and pelvis rotation angle (counterclockwise rotation + value, clockwise rotation − value), shoulder and pelvis tilt angle (posterior tilt + value, anterior tilt − value), and hip and knee joint angles (extension: + value, flexion − value).Figure 3A-D depicts the respective definitions of the horizontal rotation angles of the shoulder and pelvis, tilt angles, and hip joint and knee joint flexion angles.In addition, we measured only the horizontal rotation angle of the shoulder and pelvis, tilt (front and back), and the flexion angle of the left hip and knee joints, which is the most important factor in seoi-nage movement.A total of 13 markers were attached to avoid obscuring the 3-D view.Therefore, we limited the difficulty in collecting kinematic data and decided that the motion factor of seoi-nage was sufficient as the factor mentioned above.

Statistical analyses
Statistical analysis was performed separately for kinematic variables of the seoi-nage and performance associated with event motion.Each event was subjected to a separate independent t-test for EG and NG.For all analyses, statistical significance was set at a level of 0.05.We performed all analyses using SAS (SAS 9.1.2.software; SAS Institute, Inc., Cary, NC, USA).

Ethics approval and consent to participate
The research was approved by the Institutional Review Board of Pukyong National University.And informed consent was obtained from each subject before participation in the experiment.
The angles of flexion (−) and extension (+) of the left hip joint were analyzed when performing seoi-nage (top left in Fig. 5).As a result, in Event 2, EG exhibited an angle of 169.8° (SD = 7.34) and NG demonstrated an angle of 163.3° (SD = 13.25)showed a significant difference between the two groups (t = 2.97, p < 0.01).In Event 3, EG showed an angle of 156.4° (SD = 10.30), and NG demonstrated an angle of 146.5° (SD = 23.10), and there was a significant difference between the two groups (t = 2.70, p < 0.01).In Event 4, similar to previous results, a significant difference was observed (t = 2.26, p < 0.05); EG showed an angle of 136.1° (SD = 14.98), and NG exhibited an angle of 125.0° (SD = 30.64).
The left and right flexion angles of the hip joint when the maximum vector sum force appears when performing the seoi-nage, and the left hip joint angle was 135.7° (SD = 18.00) in EG and 125.8° (SD = 28.00) in NG, with a significant difference between the two groups (t = 2.02, p < 0.05); however, the right hip joint angle was 129.1° (SD = 18.91) for EG and 122.4° (SD = 25.14) for NG, and there was no significant difference between the two groups (t = 1.45, p = 0.150).The left and right flexion angles of the knee joint were analyzed.The left knee joint angle was 132.3° (SD = 15.22) in EG and 126.8° (SD = 33.93) in NG, indicating no significant difference between the two groups (t = 0.99, p = 0.325); the right knee joint angle was 165.2° (SD = 11.28) in EG and 136.0° (SD = 34.83) in NG, indicating a significant difference between the two groups (t = 5.36, p < 0.01) (Fig. 8).

Discussion and implications
The present study investigated the kinematic characteristics of seoi-nage in elite and non-elite athletes with skill levels.We measured kinematic patterns of reactive responses with respect to seoi-nage four event movements.The main results indicated that elite athletes (EG) showed more stable balance and force control than non-elite athletes (NG).These findings indicate that the elite athletes were leading to a flexible rotation of the shoulder and pelvis.and could be drawn nearer the body with the aid of proper back shoulder and pelvic flexion.
Seoi-nage has a greater impact the better body rotation.It is known that elite athletes rotate their bodies more during seoi-nage than non-elite athletes 1,11 , according to studies by Franchini and colleagues 13 .EG had a smaller shoulder horizontal rotation angle than NG in Event 2 at the point of maximal force development.It allegedly paid more attention to pulling inside and tilting upward than shoulder turning.The shoulder and pelvic front and back tilt angles in the overall progression from Event 2 to Event 4 showed that the rear tilt was higher in EG than in NG.In addition, the front shoulder tilt was less in NG than in EG as the ability level increased.As a result, there was insufficient back-shoulder tilt for placement and unbalancing.Moreover, the front inclination of the shoulder for seoi-nage appeared little, allowing variations to be seen depending on the proficiency induced.The movement pattern of tilting at the shoulders and pelvis is supposed to demonstrate how placement and unbalance affect movement differently.In other words, proper rearward shoulder and pelvic flexion made it easier to drag the opponent toward the body.Carrying the pulled opponent forward was made possible by the front tilt of the shoulder and pelvis 8,[14][15][16][17] .
The NG in the current study demonstrated force control and flexible their trunks less than did matched EG during seoi-nage.The reduced trunk and force control might be related to incoordination in the trunk and lower limb.The inertial force needed to place the opponent may be related to the range of shoulder and pelvic tilt.In all www.nature.com/scientificreports/events, with the exception of Event 2 for the right hip joint, the articulation angles of the hip and knee joints were greater in EG than in NG.Due to the shoulder and pelvic posterior tilt being higher in EG than NG in Events 2 and 3, the hip joint was likewise higher in EG than NG.The left hip joint was also more stretched than the left hip joint in Event 4, which is a hang phase, and the front tilt of the shoulder and pelvis was more noticeable in the EG than in the NG.Therefore the body was kept parallel to the opponent by substantially drawing them close, despite the modest curvature of the hip joint, which caused a significant forward tilt of the shoulder and pelvis.Moreover, the heel was raised to allow the body to throw while leaning forward.The hip joint flexion following placement was not similar due to the experimental apparatus.This is due to the fact that we only measured the first phase of positioning and not its completion.These results showed that elite athletes more considerable range of tilt of the shoulder and pelvis from unbalancing to placement and the moment of inertia can also change based  on the level of induced proficiency 1,18 .According to Ishii and colleagues 18 , completing seoi-nage successfully is challenging because a flexibly flexed hip joint at the beginning point of positioning did not exhibit enough angular velocity during the actual throw.Moreover, the expanded hip joint angle must be used with enough angular velocity during the early stages of placement 15,18 .
Effective seoi-nage skill requires lower limb stability and a higher-level cognitive process such as formulating the motor plan and subsequent organization of flexd step movements to environmental properties.In Event 4, EG demonstrated a more advanced knee joint angle than NG.The knee joint extension must appear to lift the opponent's center of gravity while the stance must be reduced below the knee joint [11][12][13][14][15][16][17][18][19][20] .Moreover, in all events, the right knee's flexion and extension angles were greater in EG than in NG, which is assumed to be because EG raised or pushed the opponent's center of gravity in addition to tilting and flexing the body.In Event 4, also there was a noticeable variation in the extension of EG left and right knee joints.Due to the nature of seoi-nage, the right knee extended as a result of the shoulder and pelvis rotating significantly.Some of studies, the lower right knee extension during the seoi-nage positioning phase did not demonstrate enough rotation of the shoulders and pelvis 16,21 .These findings are consistent with previous studies demonstrating that elite athletes have executed seoi-nage by sufficiently twisting the pelvis and shoulder.In addition, the left and right knee joints' flexion angles for both groups showed that the EG was more flexed or similar in Event 2 than in Event 3.
At the hip and knee joint flexion angles, EG showed a more extended pattern than NG, indicating that it looked to match the extended pattern required during the positioning phase.The maximum force vector sum in EG was 102.8 N, which was higher than in NG.This suggests that EG was more effective at pulling than NG in terms of pulling force magnitude 1,2,5,10 .In comparison to NG, EG showed higher extension in the hip and knee joint flexion angles, indicating that it behaved similarly to how extension is needed during the throwing phase.In Summary, EG had a stronger vector sum of forces than NG.Moreover, EG outperformed NG in terms of the

Conclusion
This study identified specific kinematic characteristics of seoi-nage in reactive movement with skill levels.In conclusion, the results of this study suggest that the ability of seoi-nage influences the planning of motor execution and the implementation of motor programs within expertise.Based on the data from our study, it appears that rotation angle and motion control associated with the shoulder, pelvis, hip, and knee is an important factor that a tendency to take more trunk and lower balance control during seoi-nage.As compared with other NG, EG exhibited effective control of trunk and lower motion when seoi-nage events.The results of the present study would help to guide the development of seoi-nage strategies in elite and non-elite athletes aimed to promoting their ability to trunk and lower motion to enhance in response to skill challenges.A limitation of our study is the small sample size.Further investigations with a larger sample size are needed to validate the findings of the present study.Moreover, further study is required to determine whether such a training intervention would improve seoi-nage in non-elite athletes.

Figure 1 .
Figure1.Vector analysis device.After fixing the judo vector equipment to the wall, the judo rubber tube is hung on the load cell hook to perform shoulder throw.The pulling force and direction are measured simultaneously.

Figure 2 .
Figure 2. Model diagram for each event and marker position.

Figure 3 .
Figure 3.The kinematic variant diagram.(A) Horizontal angle of rotation of the shoulder and pelvis; (B) tilt angle of the shoulder and pelvis; (C) angle of flexion-extension of the hip joint; (D) angle of flexion-extension of the knee joint.

Figure 6 .
Figure 6.The magnitude of force during maximum force appearance (absolute value, relative value).

Figure 7 .
Figure 7. Kinematic variables at the time of maximum force appearance (shoulder, pelvis).